Chemical milling process and composition



April 1961 H. s. SNYDER EI'AL 2,981,610

CHEMICAL MILLING PROCESS AND COMPOSITION Filed May 14, 1957 2 Sheets-Sheet 1 Fl G. 2

w /0 I /I IW FIG. 7

FIG. 3

NVENTOR BEN SNYDER l zywle M. ROSENBERG HERMAN 1- W. SECREST ATTORNEY Apnl 25, 1961 H. B. SNYDER ETAL 2,931,610

CHEMICAL MILLING PROCESS AND COMPOSITION Filed May 14, 1957 2 Sheets-Sheet 2 010 0J5 PARTS Tl PARTS HF ETCH RATE INVENTOR HERMAN BEN SNYDER evan/Is M. ROSENBERG MILS MINUTE 77w. Secmzsr ATTORNEY rates This discovery relates to the chemical milling of metal in an acid medium and, more particularly, to the chemical milling of aluminum, aluminum alloys, titanium and titanium alloys in an acid medium.

Prior to specifically discussing chemical milling we wish to point out the manner in which it distinguishes from pickling, brightening, decorative design and surface increase. Chemical milling may be considered to be controlled corrosion or controlled metal removal to form sculptured metal configurations. In chemical milling a relatively large percentage of the original metal may be rapidly removed so as to leave a minor amount of the original metal in a new configuration. As contrasted with this is pickling orscale removal whereby as much as possible of the oxide and other coating of the metal are removed but as small amount as possible of the metal is removed. In other words in pickling only the surface coating of the metal is removed. In brightening or surface polishing a minimum amount of the metal is removed to form a reflective surface, as the scale has been previously removed. Decorative designs are of a shallow depth, and approximately five mils of surface are removed by chemicalaction. More particularly, part of the surface is masked and the decorative design is in scribed through the masked coating. Next, a chemical solution is applied to the masked material and allowed to react to form the design. An example of this is silverware. In surface increase the object is to form a roughened surface. In order to specifically increase the surface area the same is purposelypitted by chemical action. This is done to make the surface rough enough for bonding purposes such as a metalto metalbond, for a base for paint and the manufacture of condenser plates, to name a few of the uses.

' Themechanical milling of metal is well-known and extensively used throughout the metal Working industries. Even though mechanical milling is extensively used it inherently has certain limitations. One of these limitations is the expensive capital investment for milling machines. Such a machine, capable of making large complicated configurations, may cost in the hundreds of.

atent 2,981,610 Patented Apr-L25, 1061 abrasion. For example, only casting and rough can be employed with certain metals.

In recent years, as a supplement to mechanical milling, there has developed chemical milling. One of the first patents to be issued for chemical milling was the patent to Sanz, United States Patent Number 2,739,047. Sanz describes the chemical milling of aluminum with a caustic solution. Basically, this patent presents the method of taking either a piece of fiat stock or curved stock, masking certain predetermined areas or portions of this stock with a caustic resistant covering, and immersing the masked stock in the caustic solution. The caustic solution eats away or corrodes away the unmasked areas to form the desired configuration. Naturally, it is possible to remove the stock from the caustic solution, remove the masking cover, add new masking cover and insert the stock again into the caustic solution. By such means it is possible to form intricate metallic configurations. t With grinding our discovery for the use of a strong acidsolution in chemical milling we find it possible to realize amore rapid action than Sanz realizes with a caustic solution;

to achieve a closer tolerance than the tolerance Sanzs achieves; and, to realize a smoother surface than is possible with the caustic solution of Sanz. Also, our strong acid milling solution makes it possible to work with equal ease on aluminum and its alloys be they heat treated, Work hardened, cast or forged.

In United States Letters No. 2,684,291, to Wilson et a1. there is discussed a'process for producing embossed designs on hard surface roll.

To further illustrate some of the advantages of chemical milling it is possible. to achieve the same output by chemical milling with a lower capital investment than for an equal output with mechanical milling. Also, in chemical milling only the application of masking material to the part to beformed requires semi-skilled labor as contrasted with the requirements of skilled labor for mechanithese machines inordergto do a creditable job on the;

milling of metal. Alsofbecause of cost and man-hour requirements this ty pe of milling is limited to certain configurations, namely, those configurations which are of amore 'simple or open design; However, by the skillful 'useof welding it is possible to build up these limited configurations into an intricate design. Because of the nature of the weld and the inherent weakness therein. it is not always"advisabletobuild up these limited basic configurations into an intricate design as; the weld might rupture orthe' added weight may become excessive. Furthermore, with mechanical milling there usually ret su lts a relatively rough surface. Because of excessive cost the tolerance, in a recessed arem is not a close one but must be a'rather large one such as in the hundredths of an inch.- Also, on some of the newer metals it is impractical, because of costs due to equipment, to the time of milling involved and thetechnical problems involved to mechanically mill the surfaces since the metal is so cal milling. And, more intricate configurations can be formed more rapidly by chemical milling as the metal to be milled can be immersed in the reactant solution so that all parts of the metal can be acted upon simultaneously. In a specific illustration as applied to the manufacture of airplanes it is possible to reduce the weight of certain members of the airplane. As an example, aluminum and alloy sheet stock may come in a thickness of 0.064 inch; however, from the design standpoint a thickness of 0.060 inch: may be adequate Theoriginal stock would not be reduced from the thickness of 0.064 inch to the thickness of 0.060 inch by mechanical milling as the cost would be too great. However, it is possible by chemical milling to immerse the stock in a chemical reactant solution and reduce it to the thickness desired with ease and-economy." In large airplanes this reduction thickness may mean a considerable saving in the weight of the air frame. More particularly',"for every excess pound carried by the airplane there may be required from five ten pounds .of additional gross airframe weight. Therefore, if..in 'the larger airplanes a thous.nd pounds of unneeded materialcan be saved the airframe weight by mechanical means. For example, in the case of a tapered by removing the object slowly .fromthe chemical tough and resistant tomechanical ,cutting and/or rapid waflle pattern, thexnecessary chemical milling can be relatively easily performed as the. maskingmaterial can be placed over the metal and the metal not protected by this masking material can be chemically eaten .away or chemically corroded; In a like manner: an object can be milling-solution. Also, an object having large numbers of irregularly placed recessed areas can be'readily mend-1 tacturedby; chemical milling-while;;the;.p;e aration of configuration of a chemically milled metal can be more economically achieved than a similar object by mechanical means would be time consuming and difficult to prepare.

With this in mind it is an object of this discovery to provide a method for the chemical milling of metal, examples being intricate configurations, taper, and waffle pattern.

A further object is the provision of a method for the chemical milling of a metal and, particularly, for the chemical milling of aluminum, aluminum alloys, titanium and titanium alloys in an acid medium.

A further object is to provide a method for the chemical milling at a relatively uniform and controllable rate.

An additional object is to provide a solution for the chemical milling of metal and which solution can be controlled by standard methods of chemical analysis.

Another important object is the provision of a process for the chemical milling of metal and which process provides satisfactory surfaces having well-defined edges at better than standard machine tolerances.

A still further object is the provision of a process for chemical milling resulting in satsfactory structural and mechanical properties of the milled metal.

A still further and important object is to provde for chemical milling and which results in a satisfactorily smooth surface.

Another object is the provision of a process for chemical milling'and which process produces a minimum of smut.

A further object is the provision of a process for chemical milling and which process is less expensive than mechanical milling and makes, it possible to produce more intricate configurations than mechanical milling.

In the drawings:

Figure 1 is a plan view of a curved structural member illustrating two rectangular shaped recesses in the open surface thereof.

. Fig. 2 is a longitudinal vertical cross-sectional view taken on line 2 2 of Fig. l and shows the recesses in the curved member.

Fig. 3 is a view of a mask which is placed over the non-milled structural member in Fig. 1 and protects the upper surface of said structural member, except for the two. rectangular shaped openings therein, from being attacked by the chemical milling solution.

Fig. 4 is a longitudinal vertical cross-sectional view of this curved raw material from which the structural membet; is prepared and illustrates this raw material asbeing covered with protective masking coating except where the rectangular shaped recesses are to be formed.

Fig. 5; on an enlarged scale, shows the formation of a recess in metal with the aid of a masking material covering part of the metal.

Fig. 6 is an end view of a curved structural panel prior to milling; and,

Fig. 7 is' an end 'view of the curved structural panel illustrated in Fig. 6 but after chemical milling has taken place.

Fig. 8 is a graph.

In our chemical milling solution we provide an equeous solution having a high concentration of acid, a nitrate, a chloride, a fluoride and an acetate. In addition there may be present a citrate, a phosphate and a wetting agent. Each of these components is presumed to play a role in the. chemical milling. It is to be appreciated that in solutions for milling aluminum and alloys having nitric acid in theconcentration of approximately five normal, hydrochloric acid the normal, and hydrofluoric acid two nor' mal and reaction products of these solutions with the aluminum and aluminum alloys, or solutions for milling titanium and its alloys having nitric acid' in theconcentration of five normal, hydrochloric acid two normal, andhydrofluoric acid five normal and reaction products of these solutions with titanium and titanium alloys the theoretical egplanation of the chemical reactions may bequestiona e.

.The milling solutions employed formilling aluminum and its alloys and titanium and its alloys may comprise the same components. However, we believe that some of these components function differently in the milling of aluminum and its alloys than in the milling of titanium and its alloys.

Referring now to the reaction of the milling solution comprising nitric acid, hydrochloric acid and hydrofluoric acid with aluminum and its alloys, it is to be noted that all these acids separately react with aluminum. However, in the reaction of nitric acid it is'noticed that if the acid is in the concentration range of about 100% that the reaction between the aluminum and the acid almost ceases. The cessation is probably due to the formation of a protective oxide of aluminum over the surface of the metal so that the acid cannot contact the unreacted metal. As contrasted with this in a moderated weak nitric acid solution there is not found this protective oxide and the acid is free to contact the metal. We believe that in our milling solution that the concentration of the nitric acid is sufliciently high so that there is formed on the surfaces of the metal an oxide layer. Turning now to the hydrochloric acid, this is of a special importance in the milling of alloys containing such alloying elements as zinc and magnesium. Also, the hydrochloric acid probably attacks the protective oxide on the surfaces of the metal to form aluminum chloride. In regard to the hydrofluoric acid there results between this component and the aluminum a terminal product, an aluminum fluoride precipitate.

The other components of the milling solution such as the citrate, the phosphate, the acetate and the wetting agent each play an important role. The acetate plays a vital role in decreasing theloss of the fluoride. For example, it is noticed that at temperatures above about 125 F. the loss of fluoride is considerable if the acetate is not present. However, if acetate be present the loss of the fluoride is lessened considerably. From this it is believed that the acetate combines or associates with, the fluoride in some manner to tie up the fluoride and thereby prevent its loss by vaporization. It is realized that citric acid and a phosphate, especially disodium monohydrogen phosphate, are normally used as a buffer. However, in this strong acid solution it is believed the term buffer is not applicable as the solution is so strongly acid that very little buffering action couldpossibly take place. However,'the citric acid and the phosphate are of considerable value and it is believed that this value arises from the fact that they function as a moderator. More specifically, the term moderator refers to the phenomenon of a partial deactivation of the surface such as the formation of a surface membrane over the metal being attacked. Such a membrane seems to act as. a valve or a control on the quantity of active milling agent whichcan attack the surface of the metal. In regard to the wetting agent, such as an alkyl aryl sulfonate, i.e., dodecyl benzene sulfonate, this material makes it possible to achieve a better surface. ,The wetting agent probably functions, by reducing the surface tensi'ornto allow the gases produced by the action of the solution on the metal to more readily escape from the surface and therefore to permit a more uniform concentration of milling solution to react with the surface of the metal. It is realized that if the gases do not readily escape thenthat portion of the metal covered by the gases will not be reacted upon in the same manner as that portion not covered proximately F. If the temperature is allowed to 1 decrease to a value below approximately'l00' F. and. then increased above this indicated temperature the related milling rate.

T the milling solutions we prepared, the addedpasyhydrogen'nitrate or nitric acid,,,the. fluoride action of the solution with the metal is a different one than if the solution had not been allowed to cool and then been reheated. We therefore believe that it is advisable that if the solution upon being raised to a temperature above 100 F. and reacted with metal then that solution should thereafter be maintained at a temperature above 100 F.

In the milling of aluminum and its alloys with these solutions we consider that hydrogen pickup or hydrogen embrittlement is not a problem. Actually, the hydrogen released cannot eflectively pass through the protective oxide layer on the surface of the metal to the same and to cause embrittlement.

Turning now to the reaction of the milling solution comprising nitric acid, hydrochloric acid and hydrofluoric acid with titanium and its alloys it is to be noted that of these three only hydrofluoric acid reacts with titanium. The other two acids, hydrochloric and nitric, are not capable of attacking titanium to as to be the basis of an economical milling solution. However, it is to be realized that these two acids play an important role in the milling of titanium and its alloys. Turning first to the action of the nitric acid it is noted that a solution having penetrate into a high concentration of hydrofluoric acid rapidly cor-' rodes titanium. In fact, the corrosion rate is often so rapid that a controlled product cannot be realized. However, with the addition of nitric acid a controlled product can be made. We believe that in this regard that the nitric acid reacts with the titanium to form a protective oxide or oxides on the surface of the metal. The protective oxide functions to limit the passage of hydro fluoric acid to the metal. By so limiting the reaction of hydrofluoric acid with the metal there is regulated the corrosion of the titanium. As a result there is a regu- In addition to regulating the milling rate thenitric acid acts to cut down or eliminatelhydro gen contamination. Hydrogen contaminationmay adversely affect structuralproperties of titanium and its alloys. Returning now to the oxides of titanium these react with hydrofluoric acid to form complexes of titanium,

. oxygen, and fluorine. 'These complexes or compounds are soluble in water. The value of the soluble products resides in forming better andmore uniform milledprod- '6 hydrogen acetate or acetic acid, the chloride as hydrogen cholride or hydrochloric acid, the citrate as hydrogen citrate or citric acid and the phosphate as disodium monohydrogen phosphate.

In the milling process there are a number of factors determining the milling rate. Some of these factors are temperature, concentration of components, heat transfer, evaporation, agitation, volume of milling solution to surface area of metal, and so forth. Briefly, it is readily realized that the role of the temperature is very important. Generally speaking a ten degree rise in temperature, on the Kelvin scale, indicates a doubling in the reaction rate. Therefore, it is possible to achieve with a relatively dilute or weak milling solution a milling rate at a higher temperature equal to the milling rate of a moreconcentrated solution at lower temperature.

Also, it is apparent that the concentration of the reactant plays a very vital and important role in chemical milling.

This is of importance in the fact that a control of the fuct as a solution free of a precipitate is of a more uniform concentration. Or, in other words, in a solution "having a precipitate there are concentration gradients making it more difficult to achieve a uniform product. Turning now to the hydrochloric acid, this component attacks some of. the alloying elements, viz., aluminum,

manganese, vanadium, etc., and thereby plays a vital role. 1

I will not be described again.

;. We have noticed in the milling of titanium andits alloys, as contrastedwith the milling of aluminum and its solutions, thatthe temperatures of the milling solution can be raised to above 100. F. and the solution reacted,

the temperatures lowered to room temperature and again,

raised without affecting the. reaction properties of the solution. Also, in thje'milling of titanitim and its allo ys by these solutions a smoothnessvalue ofabout R.M.S. or better, can be realized. Of cou rse,sufiicientreaction time must 'be given to smooth out the original rough surfaces. R.M.S. means root mean square" and is ex pressed in micro inches of amplitude of surface variations: Finally; as a'sidelight on the reactionof these solutions it} was]. noted'lthatTwith .the' alloy, "6A1- -4V1Ti,vthat" there was not formeda precipitate of alun'rinum. This indicatesthat 'the reaction'with titanium andits alloysfis' .Tdifferent than with aluminurnfand its alloys.

nitrate was ktratio'ri of 38%; glacial acetic acid of .a specificjgravi of 1.955.;Alao, there wenefprepared acitricacid tion and {a disodium monohydrogen' phosphate: solution. These twofsolutions were prepared by heating -waterto its fas hydrogen fluoride or hydrofluoric acid, the Iacetatejastfi hoiling ppintr andtadtling an 'exces's ofcitricacid to one milling can be obtained by control of the temperature. Heat transfer is of utmost importance as good heat transfer precludes the possibility of a local build-up of temperatures and also is essential for proper control of the milling rate. r The local build-up of temperature means that in that particular area the reaction rate will be higher and more metal will be removed or corroded away. This uneven removal of the metal can result in uneven surfaces, pitting and tapering. Evaporation is of importance in that the evaporation of a vital component, viz., such as a fluoride, means that the reaction rate will change and the life of the milling solution and the uniformity of the reaction rate cannot be as closely controlled as if evaporation had not taken place. Of great importance in the chemical milling of metal is the agitation of the solution. A well-agitated solution gives more even and uniform results than one which is not agitated. This agitation may be in the form of mechanical agitation whereby the metal undergoing the milling action is'vibrated or shaken back and forth in the milling bath. Another form of agitation may be that of the milling solution itself whereby it is agitated by a gas bubbhng through it. And, a still further and desirbale form is that of ultrasonic vibration whereby highfrequency ultrasonic agitation is applied to the solution or to the metal undergoing reaction so as to break up any localized variations in solution concentration. This also serves to avoid gas accumulation on the metal surface. Furthermore, the volume of the reacting chemical milling solution in proportion to the. surface area of the material undergoing reaction can be of considerable importance. If the volume to surface ratio is rather low it is possible that the reactive components available at the milling surface will be rapidly exhausted and therefore there will be an uneven reaction rate and possibly an. uneven removal of the metal affecting the quality of the finished product. Therefore, a large ratio of volume to surface area would probably give a more constant reaction rate and thus permit closer tolerances on the finished product. 1

To more particularly illustrate our discovery and especially the effects of the variation of the concentration of components and also theeifect of the variation 'of the temperature we herewith present specific detailed examples. .These'examples are to be taken forillustrative purposes only and? are not to ,.be"taken as limitations on theinvention. v In theseexamples the specifically enumerfateld components were utilized. More particularly, concentjrated nitric acidof aspecific gravity of approximately 1. 42 a'nd of a concentration. of; 68-70% hydrochloric acid of aj fs'pecific "gravity of approximately 1.19; and a concenand the disodium monohydrogen phosphate to the other. The solutions were allowed to cool to room temperature and the supernatant liquid decanted from the precipitate.

Saturated solutions of oxalic and boric acids were pre- "8 cates that the hydrofluoric acid' was an active agent in the milling. To solutions such' as D there was added citric acid and disodium monohydrogen phosphate to prepare solutions E and F. A comparison of D, E, and F, in an pared ina similar manner. increasing temperature range, 112-139" R, 116-152 F., 7 Turning now to specific examples of the invention and l42-206 E, respectively, shows an increase in the proper a fundamental milling solution was prepared and milling rates with an increase in temperature. Specifito this solution there were added various components to cally, D milled at the rate of about 0.9 mil per minute, make different milling solutions for testing purposes. The E at 2.0 and F at 3.0.

" Table I A n o p D E F Variable x Wt. N Wt. N Wt. N Wt. N Wt. N Wt. N 7 Percent Percent Percent;- Percent Percent Percent HNO3(6870%) 20.2 2.4 20.1 2.4 20.1 2.4 20.0 19.9 2.3 19.0- 2.3 H01 7.5 0.8 7.5 0.8 7.5 0.8 7.4 7.4 0.7 7.4 0.7 Acetlcacld (glacial) 2.3 0.4 2.3 0.4 2.3 0.4 2.3 I 2.3 2 0.4 2.3 0.4 HF(60%)- 5.1 1.7 5.1 1.7 5.1 1.7 0.4 6.4 1.9 0.4 10 n 0 04.0 04.5 64.5 04.4 04.4 01.4 Citric acid (sat'd soln) 0.4 0.4 0.4 0.4 lglllaql lliOi (satd soln) 0.2 0.2 0.1 0.1 t

T ni eraiure F 123-132 123-135 160-195 112-139 110-152 142-206 R -26 20-26 -40 20-25 22-30 0-50 1 About treated aluminum alloy such as 7075S-T6, weight or 0.045 gJin. mil.

results of these tests are presented in following Table I. In this table there are listed the components of the milling solution, the temperature of milling, the smoothness of the milled object expressed in R.M.S. terms and the milling rate. For each individual test thereis presented the percentage by'weight of eachcomponent and the normality of the stronger acids.

Referring to-Tabie I it is seen that solution A comprised nitric acid, hydrochloric acid, acetic acid, hydrofluoric acid and water. This solution was used to mill an alu- 'minum alloy such as 7075S-T6, weight of 0.045 gram 'per (square inch min-volume, in the temperature range of 123-132 F; The milling rate was about 0.7 mil per minute and the smoothness was in the range of 20 to 26. To solutions such as A there were added both saturated citric acid solution and saturated disodium monohydrogen phosphate solution to make solutions B and C. The aluminurn alloy was milled by B at a temperature in the range of'123-134 F. The comparison of B and A shows that B milled the alloy at a rate of about 0.8 to approximately 0.7 mil per minute for A. The smoothness of the alloys-6r metal milled by B and A were the same, being in the range of 20 to 26 R.M.S. A comparison of C and "B, C being at a temperature in therange of l60l95 F. and B in the range of 123-134 F., shows that C milled faster than'B about 2.0 compared'to 0.8 mil per minute. This is to be expected due to the higher milling temper- ;atureof C in comparison with B, A solution such as A wasvaried by the addition thereto of hydrofluoric acid to raise the concentration thereof to about 1.9 normal so as to make solution D. Acomparison of D and A shows that D milled in the same temperature range, at a faster Another milling solution was prepared comprising 111- tric' acid, hydrochloric acid, acetic acid, hydrofluoric acid and water. This milling solution was varied by adding 30 water inv predetermined quantities so as in effect. to dilute the concentration of the active components. Also, to solutions similar to the most dilute solution there was added additional hydrofluoric acid. The metal tested by this solution. was an alloy of aluminum having a density of about 0.045 gram per (square inch mil) volume. :The

similar solutions there were added various quantities of Watert'o make solutions B, C, D, E, and F. The active components in each of these solutions remained the same in quantity although, as realized, less in concentration. For example in solution A the concentration of nitric acid. was"2.9 normal and in solution F was 1.3 normal. For hydrochloric acid insolution A the normality was 1.0 and in solution F 044 normal, and for hydrofluoric acid in A the normality was 2.2 and in F it was 0.9; These were the limits onthe concentration of the most acidic components. The temperature range in which the chemical milling was conducted, except for solution A, was substantially the samefor all the solutions. More particularly, in solution A the temperature of the solution ranged from 145-210 F. .However, in solutionsi B through Fthe temperature ranged from approximately rate of about 0.9 instead of 0.7 mil per minute. This indi- Table II.

Table ll v A B C E v F A G 11 Variable 1 I I l Wt. Wt. Wt. Wt... Wt. Wt. Wt. Wt.

Rer- N Per- N Per- N Per- N Per- N Per- N Per- N Per- N I cent cent cent cent cent cent cent cent 'HNOa (GS-%)....- "24. 6 2. 9 H (38%) 9.1 1.0 Acetic acid (glacial). -2. 8 0. 5 6.5 2.2 H; t .57.0

MetalJ. Temperature F -210 .S 25'-40 Milling rate mils/m 4. 7

. p is heat than alloy iii-aluminum such as 7075s- 1a, weight of about eois r Referring to Table II it is seen that the chemical milling rate for these solutions decreased with the addition of Water. As expected solution A milled at the fastest rate of approximately 4.7 mils per minute, B at the rate 10 C at a temperature of aboutl50 F. As wasexpected due to the higher concentration of hydrofluoric acid solu-- tion B possessed a higher milling rate than solution A,- B being about 0.3 mil per minute and solution A about 0.2

for milling ata temperature of about 901 F. and solution of approximately 1.3, C at about 0.8 plus, D at about 0.8 mil P minule- T was so even though. solutions B minus, E at about 0.6, and F at about 0.5 mil per minute. and A were Imlllng about the Same pf f This illustrates that the addition of water to the funda- P f of the hlgher temperature C mental solution decreased the concentration of the active P P? Wlth f Showed a stronger 9 components and with this decrease in the concentration of 1111111118 r te, being n the range of about 0.7 mil per mmthe active components for a given temperature range the 10 as confpared Wlth, B havmg a rate of about 9' milling rate decreased. To solutions such as F there was mll Per i Thls Increased m1 1mg rate of.sohmon added additional hydrofluoric acid to make solutions G B to be exgected g of i .great i and H. More particularly, the concentration of the hyg edteglperatures g g t e V Q E- 5 drofluoric acid in solution F was about 0.9 normal and b giff pripare mm z SUB g or in solutions G and H it was about 1.4 normal. The teme water so as t fg i perature ranges for which the milling was done with solu- Ion i ac 50ml) 5 S f i 3 tions F, G and H was such that F was in the range Ions e concen r Ion o e m In ac sou o s D and E was about 1.0 normal, hydrochloric about 0.7 of 140443 at about 135 and H at about normal and h drofluoric about 0 8 normal Solut'on E 121 F. The milling rate for these three solutions was in comparlson wlth solutlon C, both having .a mllllng temsubstantlally the same as F and G milled at the rate of o perature of about 150 F., possessed a milling rate of about 0.5 ml per minute whlle H milled at the rate of about 0.4 mil per minute as compared wlth C of about about 0.4 ml per minute. It is seen that with the higher 0 7 mil per minute This illustrates the effect of the addi g i i of f P f m 2: g i i tion of the water so as to decrease the concentration of a a ower m1 mg i ure 0 a e components with a consequent lowering of the milling mllllng rates of these two solutlons were substantially the rate of solution B in Comparison with Solution C Turning gi it 5 3L 12: fifii ii g gg z :5 :2 2: now to solution D this solution was used for milling at a hi her con tration of h id i I temperature of 120 F., approximately lower than reg to g g p G z i is z that 2 solutions C landhE. The milling rate ofnsolutlon Dfwgs 0 30 approxlmatey t irty percent 0 t e mi ing rate 0 of e rees n tern erature an t e ower concen- Another F F of chemical S91utins was P e tra tion o f hydroliu oric acid for the two milling solutions. comprislflg 'F P fl f acid acetic acid, Also, D milled at a lower rate than E indicating the efiect fi n cum aclgl, dlsodwm y e p of temperature. These two solutions were substantially Phate, a l?- l l t a fundaiflentfil Sohmon was the same, but the one at the higher temperature milled P p dand t ls so1 utlofn was vlarled ln a number o faster To a solutlonsuch as E there was added hydrog fi t0 s trt lmmief t te gtect of t: e addltlofi1 ofhhyd r -1 chlonc acid whlch ralsed the concentration of th1s and 110m acl e 6 6C 0 emPeTa ufe uPoll e 0 6111168 from a proximately 0.7 normal in E to a roximatel 1.0 milling solution and the effect of the dilution of the con- 40 r l i l tion F, The mining tempgfature for Zhese centration of the active components. The metal used in two solutions was the same, 150 F. And, the milling 1 6 h g fullllng i i fz f b z gig alloy of rates for these two solutions were the same, approximately a umlmlm aVlIlg a eIlSl Y 0 a 011 gram Per 1 0.4 mil per minute. A comparison of solut ons E and F (square inch mil) volume. The results of this series of 1 indicates that the addition of hydrochloric: acid within tests are illustrated in tabular form in following Table 5 reasonable limits does not vary the chemical milling rate HI. by a noticeable factor. A solution such as F was modified Table III A B 0 D E F G Variable wt W W t. Wt.

Per- N Per N Per- N Per- N 1 22'- N N l e e N cent cent; cent cent cent cent cent;

HNOs 68-"0 13.3 1.3 13.0 1.5 13.0 H01 3ti% Z. 10.0 1.0 9.7 1.0 9.7 0.27 H 3.0 0.3 2.9 0.5 2.9 0.3 1.9 0.33 1.9 0.33 1.9 0.32 1.9 0.32 Citric acid 1.3 0.4 p 3.8 1.2 as 1.2 2.0 0.8 2.0 0.8 2.0 0.8 5.2 1.9

sol 1.1 1.1 1.1 NaHPO (satd o 8 0 8 0 7 soln.) 0.4 0.4 l 0.4 0.3 0.3 0.3 0.2 13 1 71.0 39.1 09.1 78.8 78.8 78.8 76.8 'r cratur "F R 150 2 Bit/lites 37 2? 4355 32-42 2.3 3% 28 52 25 5 3 303 Milling rate (mlls/ mm.) 0.2- 0.3- 0.7+ 0.3- o.3+ 0.3+ 0.8+

alloy of aluminum having a Weight of about. 0 .04 5 g./in. mil. p a Referring to the above Table III it is seen that the. to make solution G by the addition to F ofhydrofluoric "fundamental solution A comprised nitric acid at about ,acid. This raised the concentration of the hydrofluoric gogmal, hyldrtzchtljorli: (acid at abolut HOI'IItIHII and .hyldroacgid from apprgrimiagely normal in F to apfprctiximately uric act a a on norma. e me a was c emi norma 1n. e m1 mg temperatureo was apically milled at room temperature and thesoluti'on showed -proximately fifteen degrees (15 F.) less than the milling a milling rate of approximately 0.2.mil per minute. To temperature of solution F viz. F. es compared with solutions such as A there was added hydrofluoric acid to F. However, the addition of the hydrofluoric acid prepare solutions B. and C having a concentration of.hyraised thernilling rate appreciably asis seen from the fact drofluorlc acid of about 1.2normal. Solution B-was used. 75 that the milling rate for solutionG was approximately 0.8

: mil per minute as compared with about 0.4 mil per minute T1 for solution F. This Considerable increase in the milling rate of solution G indicates that the addition of hydrofluoric acid is of prime importance to the milling solution and that within reasonable limits the higher the concentraof the hydrofluoric acid from approximately 0.4 normal to approximately 1.3 normal. These three other solutions were B, C and D. As solutions, B, C and D did not diiier fundamentally from each other but as chemical tion of the hydrofluoric acid present the greater the milling 5 milling solutions they differed from each other in respect rate. to the milling temperature. More particularly, B was As a side light on solution G it was decided that other held at the temperature of about 90 F., C at a temperaalloys and metals should be subjected to the milling action ture of approximately 115 F., and D at a temperature of of this solution. Therefore, aluminum and alloys such as about 150 F. From Table IV it is seen that the milling 2014, 2017, 5052, 6061, 7075, 7178, 2024 and 1100; rate of B was less than either C or D. This was to be carbon steels such as 1025, 1090; alloy steels such as 4130, expected as the milling temperature was lower. Also, C 4340 and 4130 modified; stainless steel such as 302; and, was less than D for the same reason A comparison of heat treatable stainless steel such as 17-7PH, 174PH, and solutions B and A, it being assumed that the temperature AM350 were chemically milled. A more particular and of A was substantially room temperature, indicates that complete definition of these metals can be found in the the addition of the hydrofluoric acid increased the milling -Metals Handbook published by the American Society rate of B over A. This was to be expected. To a solution for Metals, Cleveland, Ohio. In a qualitative manner the h as c there was dd d water t ake a more dilute mllllng sob-{U011 G p y removed the metf11 and P solution E in terms of the active components. For exduced a satlfactory surf? 1ce- The m1 11111g actlon ample, the concentration of the hydrofluoric acid was deg h solution was determmed m quahtatlv? manner creased from approximately 1.3 normal to approximately whereby thedallgx a l l q for a 0.9 normal. These two solutions, C and B, were used for 55 peno g i 9 O 6 so uuon on milling at substantially the same temperatures, being in e a was Y the range of 115 to 120 F. The addition of'the water Another series of test solutions was prepared comprissubstanfan d Based th mum rate of E in co ing the components nitric acid, hydrochloric, acetic acid, i e t 1 f hydrofluoric acid, citric acid, disodium monohydrogen W1 i e mg F? was 'approxlma e y phosphate and water. The metal which was milled by md Per mmu te whlle, tha mllhng rate of C was approx various solutions of this fundamental composition of matmately mll mmute' ter was a heat treated alloy of aluminum having a density l preplared compnsmg i 9 of 0.045 gram per (square inch mil) volume, g./in. mil. hydmchlorlc acid, acetlc 301d, hydrofluoric 301d, cltrlc There were prepared seven diiierent test solutions for this acid, diSOdium y g n Phosphate, and Watef- In particular series and the results of these tests are tabuthe initial solution A the concentration of the nitric acid lated in Table IV, which follows. was relatively high being approximately 4.8 normal and Table IV A B O D E Variable Wt Per- N Wt. Per- N Wt. Per- N Wt. Per- N Wt. Per- N cent cent cent cent cent HNOe (es-%) 25.6 25.0 3.1 25.0 3.1 17.4 1.3 H01 8%) 0.5 9.2: 1.1 0.3 1.1 6.4 0.0 Acetic acid (glacial) 2.0 2.8 0.52 2.8 0.52 2.0 0.35 HF 60 1.3 3.8 1.3 as 1.3 2.6 0.87 Citric acid (satd s0ln).. 1.1 1. 0 1.0 0. 7 NazHPO; (satd soln) 0. 4 0. 3 0. 3 0. 2 H1O 59. 2 57.8 57.8 70.8 Metal. Temperature F Room 150 R. 3s-45 30-45 30-35 28-35 30-40 Milling rate (mils/min.) 0. 2- 0. 3+ 0.7+ 1.1 0L3- 1 A heat treated alloy of aluminum such as 7075S-T6, having a weight of about 0.045 gJinfi mil. 7 V

- comprising approximately 37% by weight of the solution.

The fundamental solution in this series was A which comprised a high percentage of nitric acid, being approximately 3.2 normal. The percentage of hydrofluoric acid was low being in the range of about 0.4 normal. The metal was chemically milled by solution A at approximately room temperature and at a milling rate of nearly. 0.2 mil per minute. From solutions such as A three other solutions were prepared by increasing the concentration From the fundamental solution, see A'in following Table,

solutions were employed to chemically mill :1 heat treated aluminum alloy such as7075S-T6 and havinga weight of about 0.045 gram per (square iuchmil) volume ,or 0.045 g./in. mil. A summary of the results of these milling solutions is presented in the following Table V.

. .T able V A B O D E F G H Variable Wt. Wt. Wt. Wt. Wt. Wt. Wt. Wt.

Per- N Per- N Per N .l-ier- N Per- N Per- N Pcr- N -Per- N cent cent cent cent. cent cent cent cent HNO; (68-70%) .1 37.2 4.74 36.3 4.64 '36. 3 4.04 36.3 4.64' 25.4 '25,; 3.12., 24.0 2.96 HC1'(38%) 0.1 1. 09 8.9 1.06 8.9 1. 00 8.9 1.0 6.3 6.3 0.71 9.5 1.12 Acct1caoid(glaolal) e 2.8 '0. 53 2.7 0.52 2. 7. 0.52 2.7 0. 1.9 1.9 0:35 1.8 0.34 1.2 0.42 3.6 2. 09 3.6 2.09 3.6 2 2.5 2.5 1.41 4.0 2.2 Citric (satd s'oln)' 1.0 1.0 1.0 1.0 0.7 0.7 0.7 NanHPOt (sat-d soln) 0.3 0. 3 0. 3 0.3 0. 2. 0.2 0J2 2 48.3. 4.7.0 ,47,0 47.0 62.9 62.9 59.5 1 Metal. g Temperature F 90 115 150 V 120 150 140- R 35-42 3040 30-35 I 35-45 32 42 30:45 30-40, 30-40. Milling rute-(mils/m 0. 2 0. 4- 0. 7+ 1.4 0. 2 0.4- 0. 2+ 0. 9+

:. !aheattreated aluminuin'alloysuch a 7075S-T6a'nd havinga weight oiabout 0.045 .71n. ni11. l l if As is seen fromTable V in solution A the concentration of the hydrofluoric acid was approximately 0.4 normal. The milling of the metal at room temperature was as the rate of approximately 0.2 mil per minute. To a solution such as A there was added hydrofluoric acid so as to raise the concentration of this component from approximately 0.4normal to about 2.1 normal. Three solutions were prepared in this manner, these being B, C and D. These solutions were employed for milling the indi- For example, 8 was at approximately 90 F., C atI.

cated metal and each was employed at a different temperature. about 115 F., and D at about 150? F. In each instance the solution at the higher temperature milled the metal at a fasterrate. For example, B milled the metal at approximately 0.4 mil per' minute, C at about 0.7 and D at about at approximately 120 F. and solution F at about 150 F.

were E and F. The solution E was employedfor milling For our purposes the solutions C and E can be-compared 7 as they differed mainly by E being less concentrated than C. However, the milling temperatures of these two soluhydrofluoric acid, disodium monohydrogen phosphate, oxalic acid, a wetting agent such as sodium salt of dodecyl benzene sulfonic acid and water. A, summary of these milling solutions and their reaction with aluminum is presented in following Table VI. Prior to discussing the solutions it will be stated that the oxalic acid was a saturated aqueous solution prepared by heating water tothe boiling point, adding an excess of oxalic acid, cooling, and decanting the clear supernatant liquid. -In this table it isseen that solutions A and B were substantially the same, the only differencebeing that A was used for milling at 70 F. and B at 140 F. More particularly,

, in A and B thenitric acid was approximately 0.1 normal,

the hydrochloric acid about 0.1 normal, and the hydrow fluoric acid alittle overthreenormal. The milling rate for solution A was about 0.3 mil per minute and for solution B about 2.0 mils per minute. Themetal milled was heat treated aluminum alloy such as 7 075S-T6 and having a weight of about 0.045 gram per (square inch mil) volume, 0.045 g. /in. mil. Solution A produced a surface roughness in the range of 35-40 R.M.S., and solution B a roughness in the range of 47-53 R.M.S. Turning now to solutions C and D, which werethe same in composition but were used for milling at two difierent temperatures, it is seen that these solutions comprised about 0.1 normal nitric acid, hydrochloric acid being about 2.2 normal and hydrofluoric acid being about 3.2

tions were substantially the same, about 115 to 120 F. .1 7

It is seen by this comparison that E milled at a lower rate than C, approximately 0.2 mil per minute to approximately 0.7 mil per minute. This means that the addition of the. water substantially lowered the milling rate. further evidence in this regard a comparison of solutions Fand D indicates the same, as these two solutions were substantially the same except that F contained more water than D and therefore the. concentration of the active components was less. Again, the milling rate of the more dilute or less concentrated solution i s less than the milling rate of the concentrated solution. For example, the solution F milled at approximately.0.4 mil per minute while thesolution D at. about 1.4 mils per minute. The effectiveness of the concentration of the hydrochloric acid was brought out in the comparison of solutions G and E whereby solution G was substantially the sameas solution E except that G contained more hydrochloric acid.

andH, wherein .the hydrofluoricacid concentration ofH H p was raised over that of G from about 1.4 normal to about 2.2 normal.

,niilled at arate of about 0.9 mil per minute while solution G milled at a rategoj approximately 0.2. mil perj minute. -At the concentration of the components in the "solutions G and H it is seen that the higher the concenya .tration of the hydrofluoric acid presentthe greate t chemical milling rate.

Another series of chemical milling solutions for the of aluminum was prepared. This series of solu-q The temperatures of; these two solutions 7 were approximately the same, being in the range of about 140 to 135 F. However; it is noted that solutioniH normal. "Solution C was used for milling at F. and had a milling rate of 0.8 mil per minute while solution D was used for milling at F. and had a much higher milling rate of4 mils per minute. Solution C gave a better surface than D, having a roughness value in the range of 36-40 R.M.S. and. D a value in the range of 50-55, R.M.S. A comparisonof C with A shows that for anincrease in the concentration of the hydrochloric acid with the concentration of the other components remaining substantially the same that the milling rate increased. In fact, for the same temperatures the milling rate of C was over twice as great as the milling rate of A, and even for a decrease in temperature of D with respect to B the milling rate of D was approximately twice the 'milling rate. of B. I Thisindicates that there should befa minimum concentration of hydrochloric acid present to cut the alloying elements. Below this minimum concentration the millingrate is depressed. Another set of solutions was prepared, E and F, wherein the concentration of the nitricacid was raised to 3.1 normal, the concentration of hydrochloric was 0.1 normal, and hydrofluoric was. decreased to 1.6 normal. Composition E was used for milling at 100 F. and milled at the rate of 0.1 mil per minutearid solution F was used for milling at F. and milled ati the rate of 1.3 mils per minute. The R.M.S. value of E was in the range of 28-33 whilefor F was in .therange of 41-47.. A comparison of E with A, B, C and D indicatesthat withanincrease inthenitric acid concentration that the milling rate was decreased somewhat. However, there was a decrease in the concentration ofi the hydrochloric and hydrofluoric acid which may, also be interpreted to mean that the milling rate decreases with the decrease in the concentrations of these components. A comparison of E and F shows that with a sufficiently high milling temperature the effect of a low concentration of components can be overcome so as toachieve a satisfactory milling. rate.

Another series of milling solutions was formulated "j and in this series the concentration, of the hydrofluoric acidwas varied-from zero to approximately 2.7 normal, For this. series of milling solutions .see, following Table tions comprised nitric aci d, hydrochloric acid, acetic acid, 75 VII.

Table VI A B s O D E 13 Variable Wt. N Wt. N Wt. N Wt. N Wt. N Wt. N Percent Percent Percent Percent Percent Percent HNO; (68-70%) 1 1 l 1 0.1 25 3. 1 25 H01 8 1 l 20 20 2.2 1 0.1 1 10 10 2 2 0. 4 5 0. 9 5 10 10 10 10 3. 2 5 1.6 5 N azHPOr (satd soln) 0.2 0. 2 0.2 0. 2 0.2 0. 2 Oxallc (sat'd soln) 0.6 0. 6 0.6 0.0 0. 6 0. 6 Wetting agent". 0. 1 0. 1 0. 1 0. 1 0. 1 0. 1 H 77. 0 77. 0 66. 1 66. 1 63. 0 63. 0 Metal. Temperature F 70 140 70 120 100 160 Milling rate (mils/min)-.. 0. 3- 2 0+ 0. 8+ 4. 0+ 0.1+ 1 3- R.M.S -40 47-53 36-40 -55 28-33 41-47 {A heat treated aluminum alloy such as 7075S-T6 having a weight of about 0.045 g. /in. mil.

Table VII A B O D E F G Variable Wt. Wt. Wt. Wt. Wt. Wt. Wt. Per- N Per- N Per- N Per- N Per- N 'Per- N Per- N cent cent cent cent cent cent cent HNOa (68-70%) 20. 4 2. 5 20.3 2. 5 20.0 2. 4 19.2 2. 4 18. 8 2. 3 18. 8 2. 3 18. 8 2. 3 Acetic acid (glacial) 2. 3 0. 4 2 3 0. 4 2,. 3 0.4 2. 2 0.4 2. 1 0. 4 2. 1 0. 4 2. 1 0. 4 full (38%) 16.4 1. 9 16. 3 1. 9 16.1 1.8 15.5 '1. 8 15.1 1.8 15.1 1. 8 15.1 1. 8 HF 0 0.9 0. 25 2. 5 0. 76 6.1 2.0 8. 0 2. 8.0 2. 65 8. 0 2. 65 Citric acid. (sat'd sol'n) 0.9 0.9 0.8 0.8 0.8 0.8 0.8 NmHPO; (satd soln) 0. 3 0. 3 0. 3 O. 3 0. 3 0. 3 0. 3 1131101. 59.6 59.1 58.2 56.0 54.8 54.8 54.8

eta Temperature F -137 135-132 132-134 133-138 133-136 -152. 5 121-122 Milling rate (mils/ min.) 0 0.10 1.7 3.8 '3.8 4.8' 1 8 V 15 15 13 1s 15 1 6Al-4V-Ti, an alloy of titanium, aluminum and vanadium. Fundamentally, the solutions of Table VII comprised nitric acid, acetic acid, hydrochloric acid, citric acid, disodium monohydrogen phosphate and water. The alloy upon which the chemical milling solutions were used was used for milling at different temperatures. Solutions E, F, and G were substantially the same. More particularly, solution F was used at a temperature of approximately 150 F. and solution G at a temperature Temperature 5. R.M.S

I Milling rat e -4--.

'Ti-Very fast; 17- 7PH- Fair-good.

an alloy of titanium, more specifically, C-lZOAV, and 40 of about 121 F. Contrasting the three solutions, E, F generally referred to 6Al--4V-Ti. In solution A there and G, it is seen that F milled more rapidly than E,'4.8 110 hydrofluoric acid. allfl g temperature mils per minute to 3.8 mils per minute. Also, G milled was about 140 F., and the Il'lllllIlg rate was substantially more slowly h i h E or F at the rate f 1 3 mils zero. In solutlonsB, C, and E the concentration of per minute, These figures are as expected as with the the hydrofluoric acid was increased to 0.25 normal, 0.76 45 increase of temperature of F over'E and G there was an 110831211, normal and 1' l resphectlvely' The increase in the milling rate for substantially the same 7 yi i ggf g z l m f g g lp fundamental solution. And, for a decrease of temperatl lz millihg rate incre as zd xilli n inc ase iil ille :83 G and there was a decrease in the mining icentration of the hydrofluoric acid. More particularly, 50 rate In m It can be a1d.that mcreae.m for solution B the mining rate was 0 10 mi] per minute the hydrofluoric acid concentration increases the milling for C a mining rate of L7 for D a rate of 3.6 and rate and an increase in temperature increases the milling E a rate of 3.8. The surface roughness or R.M.S. values rate V was approximately the Same in the range of Turning now to another series of solutions comprrslng Again, this progressive rise in the milling rate with the 55 the P P P t QC1 11Y 1'0fIh1O11C ac d, acetic acid, progressive increase in the concentration of the hydrohydrofluoric acid, cltllc 6 s nr m nchy r gp fluoric acid indicates that the hydrofluoric acid plays p p e and water, the results f c c m l mi ling i a very important role in the chemical milling; As this Series Of o uti s are p e n ill h owing a variation on solution B solutions F and G were Table 'VIII. 7 e

' i TableVlll A V B o V 1) Variable 1. v Wt. Percent N Wt. Percent N Wt. Percent N Wt. Percent IN HNOa (68-70%) 23. a 22. 7 2. 9 2c. 2 2 2 H01 3s% 5. 2 .91.] 1. 2 8.8 0.9 Acetic acid 2. 7 2. 3 0. 5 2. 3 0. 4 HF 50 9.5 9.0 3.1 15.5 5.0 Citric acid (satd sol'n) 1.0 1.0 0.8 NEQHPO! (satd soln)... 0.3 0.3 0.3 57.4 54.5 48.6

HBr(47%) 2.4 n

T1 17-7PH.316 '11, 17-7PH; 316 Tl, 17-7PH; 316.

Room- Room.

Ti-Extremely fast; 17-7 PH-Fast 316- Fair.

Ti-Extremely last;

l7-7PH-Fast; 316-Fast.

Ti-Extrerncly fast; 17-7PH Fast; 3l6-Fast. a

, covered from 2.0 normal to about 5.1 normal.

17 In Table VIII solutions A and B are comparable as they are substantially the same solutions with only the milling temperature difiering, i.e., for A the milling temperature was 80' F. and for B the milling temperature 18 the indicated metal. Then, to this solution'there was added additional hydrofluoric acid to make solution B and additional metal milled. This was repeated for solutions C, D, E and F. Asis readily realized there was was 160 F. The metals milled by these solutions were a buildup of the reaction products between the metal and commercially pure titanium, 17-7PH steel and 316 steel. the reactant SOhItiOII- in effect there were 50 y Solution A milled titanium very rapidly and milled 17 parts of the alloy in solution for each successive addition 7 PH steel rapidly although not as rapidly as the titanium. 0f thohydmfllloiic add to ma e a new Solution Refer- However, with an increase in the temperature from 80 Ting again to Solution A it i en t t at e tart of this to 160 F. solution B milled titanium extremely fast, 10 reaction there were zero parts'of themetal in solution milled 17-7PH steel fast, and reacted fairly fast on 316 d at the end of the reaction there was 0.005 part per steel although not as rapidly as on the 17-7PI-I. Studyp of hydrofluoric acid present The P of hydroing the results of milling with A and B it is seen that with fl i a present was based on the volume of the acid. an increase in temperature the milling rate increased make p rcent by weight (60%) and the specific gravity ing it possible to more rapidly mill some metals at higher of the acid-V The Weight of metal dissolved was temperatures which would be practically impossible, ecokn wn and therefore the ratio of part of metal to part of nomically speaking, to mill at lower temperatures. Soluhydrofluoric add Was i y detel'mihedwith 115561" tion 'C was a modification of solutions A and B in that ence to the succeeding solution, for example, solution'D, the concentration of the hydrochloric acid was increased t the Start Of the milling reaction there were about 0-029 at the expense of the nitric acid, acetic acid, hydrofluoric 20 part Of the alloy i he Sol tio a d at e end of the mi acid present. The alloys milled by solution C were coming reaction there were about 0.033 part of alloy in the mercially pure titanium, 17-7PH steel and 316 steel. solution per part of the hydrofluoric acid. The tempera- Again, solution C milled titanium extremely fast, and ture of milling solution was substantially at 130 F. or, milled 17-7PH and 316 steel fast although not as rapidly more particularly, 130i2 F. The milling rates of as the titanium. Solution Dcomprised the same comthese solutions were not identical but were in the same ponents as solutions A, B and C, but the concentration range. For example, solution A initially had a milling of the nitric acid was considerably lower being 2.2 n0rrate of 1.9 mils per minute and decreased to 1.0 mil per mal, the hydrochloric acid was 0.9 normal, the acetic minute with buildup of the metal-solution product in the acid was substantially the same as for the other solutions, solution. Solution B had an initial rate of about 3.1 mils but the concentration of the hydrofluoric acid was inper minute and decreased to 2.4 mils per minute. Solucreased to 5 normal. This solution was used for milling .tion C started out at approximately 1.8 mils per minute titanium, 17-7PH and 316 steels. With the titanium the and decreased to 1.2 mils per minute. Upon further adsolution milled extremely fast, with 17-7PH steel the sodition of hydrofluoric acid, solution D initially had a milllution was fast, and with 316 steel the solution was also ing rate of 2.3 mils per minute and decreased'to 0.6 mil fast. Referring again to Table VIII it is seen that the per minute. Similarly, solution B had an initial rate of milling rate in this table is given in terms of a qualitative 2.3 mils per minute and decreased to 1.2, and solution F value and not in terms of a quantitative value. For this had an initial rate of 2.1 mils per minute and decreased particular series of solutionsthe change in weight of the to 0.7 mil per minute.

Table IX I A B 0 D E 1 Variable Wt. Wt. Wt. Wt. Wt. Wt.

Per- N Per- N Per- N Pcr- N Per: N Per- N cent cent cent: cent cent cent HNOS (68-70%) Acetic acid (glaclal) H01 (38% Temperature F Mllllngrate mils/min Start of reaction-part metal in soln per art of soln.

End 0 reactionpart metal in soln per part of soln.

metals milled was not determined. Therefore, a qualitative determination of the milling rate was used. From this table it is seen that an increase in temperature increases the milling rate, that an increase in the concentration of the hydrochloric acid and the hydrofluoric acid present also increases the milling rate.

Another milling solution was prepared, see following'165 Table IX, comprising nitric acid, acetic acid, hydrochloric acid, hydrofluoric acid, citric acid, disodium monohydrogen phosphate, and water. The fundamental solution was varied by increasing the concentration of the hydrofluoric acid present so that the range of hydrofluoric acid- 70 More particularly, in this series of'milling solutions a titanium alloy, said alloy being 6Al--4VT i, was milled by these solutions. The weakest solution with respect-to the hydrofiuoric acid present, see solution'A, was .used "to mill Additional information was found for solutions A and F in that the milling rate for the various parts of metal in the solution per part of the hydrofluoric acid originally present was determined. This information is recorded in column form in Table X. Referring to the table it is seen that for solution A with no metal dissolved in the solution the initial milling rate was approximately 1.63 mils per minute. Upon the dissolving of the metal in the solution the milling rate momentarily increased to 1.88 and then leveled off at approximately 1.75 mils per minute until the concentration of the products in the solution reached approximately 0.05 part of metal per part of hydrofluoric acid. After this time, and also upon the increase in concentration of the metal, in the hydrofluoric acid, the reactionrate decreased to 1.67 mils per minute or approximately 1.7, then decreased to 1.5, 1.25, and finally to about 1.0mi=l per minute. Turning now to solution F it is seen that with a concentration of the metal in the solution of approximately 026 part metal to each part of hydrofluoric acid the milling rate was about 2.1 mils per minute. With the buildup of the concentration of the metal to about 0.3 the milling rate was about 1.1 and with a further buildup of the metal the milling rate decreased to about 0.75. A plot of these milling rates is in Figure 8. From this graph it is seen that for solution A there is a rather long and uniform milling rate based upon the buildup of the metal in the solution. However, for solution F it is noticed that the reaction rate decreased rapidly. From this information it is hypothesized that the concentration of the weak acid in the solution such as the combination of the acetic and the citric acid determines to a large degree the uniformity of the milling rate of the solution. For example, with solution A the ratio of the volume of the glacial acetic acid and the saturated citric solutions to the total volume of the solution was 0.032 part per part, and for solution F this same ratio was 0.022 part per part. From this it is possible to state that the presence of 'a Weak acid such as citric acid, acetic acid, oxalic acid, and boric acid having a primary ionization constant in the range of to 10- moderate the reaction so that it assumes a longer uniform rate of reaction. Below a certain volume concentration of these indicated weak acids the reaction starts out at a high rate and rapidly decreases to a low rate. Above a certain minimum concentration of these acids the reaction rate of the solution was relatively uniform and instead of precipitously decreasing the reaction rate decreased slowly. In fact, the reaction rate may be considered to be on a plateau for a long period. Naturally it is readily realized that an advantage of this type of reaction is the closer controlling of the same. With the uniform reaction it is possible to control the reaction and the eating away of the metal so that a more uniform product can be realized. As contrasted with this a reaction which starts out at a high rate and rapidly decreases to a low rate is more difficult to control so as to predict the time period in which the metal should be immersed in the solution in order to achieve the proper amount of metal removal.

Norm-Ratio oi acetic plus citric acid solution to total volume of solution equals 0.032.

SOLUTION F [Parts by weight HF=15.1%]

Milling Parts at Parts Rate Start Metal/ mils/min. Parts HF NOTE. -Ratio of acetic plus citric acid solution to total volume 01- solution equals 0.022.

Table XI A B Variable V wt. Percent N Wt. Percent N HNO: (68-70%) Acetic acid (glacial) H01 (38%) i H H e ps-P e i w MNMNIO'MJINM Ti, 17-7PH; 6Al4V- 6Al4V-Tl; Ti, 17- T1. 7PH, AM350.

Temperature F Milling rate Ti and 6Al4V-Ti- Very rapid; 17- 7PH-' Rapid.

RMS

' Table XI summarizes in a qualitative manner the reaction rate of a typical solution on various metals and also the R.M.S. value of these metals both before and after milling. The solution comprised nitric acid, acetic acid, hydrochloric acid, hydrofluoric acid, hydrobromic acid, citric acid, disodium monohydrogen phosphate and water. The presence of the hydrofluoric acid was in the range of approximately 5.3 normal, the nitric acid about 2.4 normal, and the hydrochloric acid about 1.0 normal. At 80 F. the solution, see A, reacted very rapidly with pure titanium and an alloy of 6Al4V-Ti, and reacted rapidly on l7-7PH steel. At 160 F. the same solution reacted at a good rate with AM350 steel, and reacted very rapidlywith pure titanium, 17-7PH steel and 6Al4V- -Ti. In regard to the R.M.S. values for titanium the as rolled value was in the range of 50 to 60 while the milled metal value was in the range of 8 to 11. For 6Al4V--Ti the as rolled values were in the range of to while the milled metal value was in the range of 4 to 6. And, with l7-7PH the as rolled value was about 50 while the corroded value was 45. This reaction solution produced a surface on metal which was extremely smooth, for example, see titanium and the alloy of titanium having values in the range of 8-11 and 4-6 respectively. The 17-7PH milled metal was also somewhat smoother than the as rolled metal.

In Table XII there is presented another fundamental milling solution with variations thereof. Fundamentally, this solution comprised nitric acid, hydrochloric acid, acetic acid, hydrofluoric acid, and water. The nitric acid was about 4.7 normal and the hydrofluoric acid about 1.4 normal. At approximately room temperature solution A reacted very rapidly with 17-7PH. To this solution there was added citric acid and disodium monohydrogen phosphate to form solution B. Solution B was used for milling 17-7PH steel and it was found that fast milling was achieved with this solution. However, it was observed that B did not mill as rapidly as A, This indicates that the addition of the citric acid and disodium monohydrogen phosphate. moderated the milling action. This moderating action comprised slowing down the reaction rate so as to more closely regulate the reaction. To a solution such as solution B there was added additional citric acid and a complex salt of ammonium fluoride and hydrogen fluoride. As contrasted with solutions A and B solution C was used to mill both 17-7PH and a rapid millingjob.

Table XII the milling rate 0.5 mil per minute, and the R.M;S. value about 12. Again, acomparison of B with B, or approxi- A B C mately same milling temperatures and with an increase Variable in nitric acid concentration and a decrease in hydrofluoric w w w concentration there was apparently a synergistic effect 55;; 5;; N 5 N tending to lower the milling rate of the solution. And, in solution B the nitric acid was about 3.9 normal, the 36 7 4 7 6 4.7 hydrochloric about 0.6 normal and the hydrofluoric about 9.0 1.1 8.8 1.1 5.2 normal. The milling temperature was 90 F., the 2 ii 3 2 2 milling rate 2.0 mils per minute and the R.M.S., 9. A M comparison of E with C, whereby the two were at sub- Oltrlc acid (satd soln.) NHQHPOL (sat'd soln.) stantlally the same mlllmg temperature, shows that even atggggg -g thoughthere is an increase in the nitric acid concentrai1 1 tion that if there is a suflicient increase in the hydrofilgfifi 17:71:11 353 3551 5 fluoric acid concentration there will then be a rapid mill- 4V-Ti. ing rate. An overall view of these compositions, A, B, p C, D, and E indicates that with the raising of the concen- Table XIII A B o D E V rlable a Wt. Wt. Wt. Wt. Wt.

Per- N Pcr- N Per- N Per- N Per- N cent 1 cent cent; cent 1 cent HNO 6870 s 0.9 10 1.2 3.9 Kati 2 a 2 n l m o ii iuu. s 2.5 2 0.6 15 5.2 NH4F.HF 2 .4 NQZHPO4(S8tdSO1IL). 0.4 0.4 0.4 Borlc acid (satd soln.). l. 2 glitrlic acid (satd soln.) 6.1 t w H di??? 74.2 71.1 43.2 Temperature F 90 135. 90.

rate (mils/m 15 10 963 (1125. 5.0. MQtal'IIIIIIIIIII eA'i 'lv-TL: 6Ai-4v-Tl..I 6A1-4v-TLII 6A I-4VIi GAliV-Tl.

Another set of milling solutions for the milling of titanium was prepared (see Table XIII). These solutions comprised nitric acid, hydrochloric acid, acetic acid, hydrofluoric acid, amoniurn bifluoride, disodium monohydrogen phosphate, boric acid, citric acid, a wetting agent such as the sodium salt of the sulfonic acid of dodecyl benzene, and water. The boric acid solution was prepared by heating water to the boiling temperature, adding an' excess of boric acid, cooling and decanting the saturated boric acid solution. The metal milled by the solutions was a titanium alloy such as 6Al--4V--Ti. Solutions A and B were the same except that solution A was used for milling at 80 F. and B at 120 F. In these solutions the concentration of nitric acid was 0.1 normal, the hydrochloric 1.1 normal and the hydrofluoric about 3.1 normal. The milling rate for solution A was 0.5 mil per minute and for solution B about 2.5 mils per minute and the R.M.S. value for A was 15 and for B, 10. It is seen that a relatively modest increase in temperature from A to B in a solution comprising a relatively high concentration of hydrofluoric acid produced a large increase in the milling rate. Attention is called to the fact that nitric acid concentration was rather low, 0.1 normal, in solutions A and B. In solution C the nitric acid concentration was raised to about 0.9 normal, hydrochloric lowered to 0.5 normal and hydrofiuoric acid decreased to 2.5 normal. The milling ternperature was 90 F, and the milling rate 0.3 mils per minute, the R.M.S. value 16. A comparison of C with .A shows a decrease in the milling rate. It can be surmised that the corresponding increase in the concentration of the nitric acid and the corresponding decrease in the concentration of the hydrofluoric acid produced a synergistic effect so as to lower the milling rate of solution C with respect to solution A even though the temperatures were approximately the same. In solution I) the nitric acid concentration was about 1.2 normal, the hydrochloric acid 1.1 normal and the hydrofluoric about 0.6 normal. The milling temperature was 135 R,

tration of the nitric acid and the lowering of the hydrofluoric acid concentration there is a decrease in the milling rate. However, if both the nitric acid concentration and the hydrofluoric acid concentration be raised, particularly the latter, there is an increase in the milling rate.

Having described our discovery and illustrated the same with reference to various compositions of matter for the use of chemical milling solution, reference is hereby made to the practical application of these solutions. Referring to the drawings, and especially Figure 1, it is seen that there is illustrated a structural panel 10 having two chemically milled recesses 11 therein. In Figure 2, a vertical cross-sectional view of Figure 1, it is seen that this structural panel is curved and that these two recesses are separated from each other by a rib 12. In the manufacture of this type of structural panel the material to be saved is covered by an adhering and strippable protective film or coating, there are numerous such coatings or films examples being neoprene, vinyl etch-proof film, and others. More particularly, for shallow milling there may be used various acidresistant paints. To pictorially illustrate the making of this structural panel reference is made to Figures 3 and 4. In Figure 3 there is illustrated a neoprene mask 13 having two recesses 14 therein. This neoprene film 13 is placed on the concave side of the structural panel 10 and on the edges and the convex side are attached other protective neoprene films. A cross-sectional view of the structural panel before chemical milling takes place is shown in Figure 4 whereby the panel is covered by the neoprene having the two cutouts 14 illustrated therein. This structural panel is now ready to be chemically milled and is dipped or submerged in the milling solution for a predetermined time. With the passing of the time the milling solution eats away or corrodes away the metal so as to form the two recesses 11. It is to be noted that that portion of the metal covered by the protective neoprene fihn is not attacked and only that portion of the metal not covered by the protective film is corroded away. After the passage ofapredeten mined period of time and the recesses are of a sufiicient depth and size the structural panel is removed from the milling solution. The adherent solution is washed away and the neoprene protective film stripped off the panel to produce that shown in Figure 1. Other teachings for the masking of metal may be found in United States Patents Numbers 2,739,047 to Sanz and 2,684,291 to Wilson et al.

In Figure there is illustrated the action of the milling solution on the metal at the junction of the protective film and the metal. It is seen that milling solution 16 cats away metal 17 so as to undercut protective film 18. One ofthe particular advantages of the use of a wetting agent is to induce better contact between the milling solution and the metal. With a better contact or a lowering of the effective surface tension there is less possibility of gases collecting on the surface or near masked edges underneath the protective film. For this function there are a number of suitable wetting agents such as pine oil. One of the common wetting agents is an alkyl aryl sulfonate. Specific sulfonates which are useful are the sodium salts of dodecyl benzene sulfonic acid and pentadecyl benzene sulfonic acid.

Another instance where our solution is of value is in the chemical milling of components for aircraft. In Figure 6 is illustrated a curved structural plate having a thickness of 0.61 inch. This plate was transformed into a structural member, for use in a fuel tank, having a thickness of 0.100 inch, outside dimensions of eight inches square, a circular boss in the center of three inches in diameter and of the original thickness of 0.610 inch. The finished product was prepared from the original structural plate by masking the central portion on each side so as to form the boss upon the corrosion of the rest of the metal. Normally, such a metal would be formed by fabrication using welding techniques. The welding process causes grain growth which in turn causes a weakening of the metal. With our method there is no grain growth.

The control for determining the degree of chemical milling may be carried out in a number of different manners. One of these is to introduce a material of a predetermined thickness into the chemical milling solution along with the member to be chemically milled and to periodically weigh or measure this member. When this member has decreased a certain amount in weight or thickness it is possible to decide whether the structural member undergoing chemical milling has been corroded away sufficiently.

Returning now to the tables and the examples presented therein it is noticed that the smoothness or R.M.S. values are in the range of approximately 20 to R.M.S. This is of a special importance as this value represents a surface that is relatively smooth. In fact, the milled metal is often smoother than the metal when machined milled. In machine milling the average smoothness value is approximately 125 R.M.S. Of course, this value of 125 R.M.S. is subject to variation as with soft alloys it is possible to machine mill the metal somewhat smoother than 125 R.M.S., but with heat treated, work hardened, and tempered alloys which may have a tough surface skin it is difiicult to achieve a smoothness value of 125 R.M.S. on machine milling. However, with this chemical milling process it is possible to achieve a smoothness value which approaches that used for reflective surfaces, and it is possible to achieve this smoothness on all types of alloys be they hard, heated treated, work hardened, or tempered.

The smoothness values were determined by a profilometer. The profilometer used possessed a diamond needle which rode on the surface of a metal and measured the profile of the surface in millionths of an inch in amplitude. These measurements are expressed in R.M.S. or root mean square values. Such an instrument is commercially obtainable from Physicists Research Co.,

Ann Arbor,- Michigan.

In this chemical milling process it is noticed that there is little if any smut. Smut is defined as that material which clings to the metal itself. Smut is usually an oxide of the alloy or chemical compounds of the alloy or unreacted finely divided alloying materials and is attracted to the bulk metal by electrochemical and electrostatic forces. 7

With our process and chemical milling solution it is possible to achieve tolerances in some usages which have not been attainable in commercial practice before. These tolerances are in the range of approximately plus or minus 0.0005 inch, making it possible to achieve a high quality and/or a precision product. Also, in our process it has been observed that there is no intergranular corrosion due to the chemical action of the solutions. This success in not having intergranular corrosion makes it possible to achieve higher strength components for less weight of the component as compared with the presently machined milled and welded components.

Our process is also adaptable for a continuous method or a batch method. In the continuous process it is possible to contact the structural item to be made with a milling solution and to continuously recirculate the milling solution so as to maintain uniformity of solution in contact with all reacting surfaces. Chemical processing equipment, i.e., tanks, pipe fittings, valves, and pumps lined with a suitable resistant material such as polyethylene, polyvinylchloride, and polyvinylesters, capable of withstanding the chemical action of these solutions are available. Also, filter screens and heat exchange equipment to withstand the chemical action of these solutions are available. Therefore, it is possible to maintain close solution control necessary for continuous operation.

Examples of other ways and means for chemically milling metals are herewith presented. A metal may be re-' moved by spraying the solution onto it so as to have a fine stream of the reactant solution contact the metal. By this technique deep grooving can be realized by rotating a part. In another manner a pipe may be made lighterin-weight by running the solution through it so as to'decrease the wall thickness. As is realized the interior wall of the pipe is eaten away and the inside diameter thereby enlarged.

From the above description of our discovery it is seen that the same is applicable to the chemical milling of metal and alloys. From our discovery it is possible to save considerable weight in the manufacture of metal products; to produce the parts in batches or continuously; the capital investment for production of the components may be as low as approximately five to ten percent of the capital investment for equivalent machine milling. Also, it is possible to produce highly complex shapes and configurations with chemical milling which are not possible with machine milling, to use non-symmetrical patterns with chemical milling which are diflicult to use with machine milling, to produce integrally stiffened structures whereby one unit serves as the structure instead of an integrally fabricated structure depending upon welds, which are weaker than the metal itself, to produce the structure, and to make tapered structural materials which are difficult to make with machine milling. Furthermore, from an engineering design standpoint it is seen that it is possible to etch after forming the structure. In many operations it is economically feasible to obtain closer tolerances, to have a number of various depths of cut, and to mill all surfaces of the part simultaneously. In this regard, it is possible to chemically mill with equal case all types of aluminum and titanium and their alloys. From a labor standpoint and skilled artisan viewpoint it is not necessary in chemical milling to use as highly skilled operators as in machine milling.

Having described our discovery what we claim and wish to protect is as follows:

1. A composition of matter adapted for chemically milling metals which consists essentially of an aqueous acidic medium containing a nitrate in the concentration range of about 0.1-4.7 molar, a chloride in the concentration range of about 0.1-2.2 molar, a fluoride in the concentration range of about 0.25-5.3 molar, and an acetate in a concentration of at least 0.22 normal, the aqueous medium as a whole having an available hydrogen ion concentration in the concentration range of about 2.8-10.7 molar.

2. A process for chemically milling a metal selected from the group consisting of aluminum, titantium, steel and alloys of these metals, said process comprising subjecting these metals to the action of a composition of matter which consists essentially of an aqueous acidic medium containing a nitrate in the concentration range of about 0.1-4.7 molar, a chloride in the concentration of about 0.1-2.2 molar, a fluoride in the concentration range of about 0.25-5.3 molar, and an acetate in a concentration of at least 0.22 normal, the aqueous medium as a whole having an available hydrogen ion concentration in the concentration range of about 2.8-10.7 molar.

3. A composition of matter adapted for chemically milling metals comprising an aqueous acidic medium containing a nitrate, a chloride, a fluoride, and minor amounts of a phosphate and a weak acid having an ionization constant in the range of to 10 the concentration of the nitrate and the chloride each being at least about 0.1 molar, the concentration of the fluoride being at least about 0.2 molar, and the aqueous medium as a whole having a available hydrogen ion concentration at least equivalent'to about 2.8 molar.

4. A composition of matter adapted for chemically milling metals comprising an aqueous acidic medium containing a nitrate in the concentration range of about 0.1-4.7 molar, a chloride in the concentration range of about 0.1-2.2 molar, a fluoride in the concentration range of about 0.2-5.3 molar, and minor amounts of a phosphate and an acetate in a concentration of at least about 0.22 normal, the aqueous medium as a whole having an available hydrogen ion concentration in the range of about 2.8-10.7 molar.

5. A process for chemically milling a metal selected from the group consisting of aluminum, titanium, steel and alloys of these metals, said process comprising subjecting these metals to the action of an aqueous acidic centration of hydrofluoric acid being at least about 0.2 molar, and minor amounts of a phosphate and a weak acid having an ionization constant in the range of 10- to 10*, the aqueous medium as a whole having an available hydrogen ion concentration in the concentration range of about 2.8-10.7 molar, and maintaining the tem perature of treatment at between about room temperature and 210 F.

i 6. A composition of matter adapted for chemically milling a metal selected from the group consisting of aluminum, titanium, steel and alloys of these metals, and which consists essentially of an aqueous acidic medium containing a nitrate, a chloride, a fluoride and an acetate, the concentration of the nitrate and chloride each being at least about 0.1 normal, the concentration of the fluoride being at least about 0.25 normal, the concentration of the acetate being at least about 0.22 normal, the available hydrogen ion concentration being, however, at least 2.8 normal.

7. The composition of claim 6 wherein minor'amounts of phosphate and citrate'are present in said composition.

8. A process for chemically milling a metal selected from the groupconsisting of aluminum, titanium, steel and alloys of these metals, said process comprising subjecting the metal to the action of a composition consisting essentially of an aqueous acidic medium containing a nitrate, a chloride, a fluoride and an acetate, the concentration of the nitrate and chloride each being at least about 0.1 normal, the concentration ofthe fluoride being at least about 0.25 normal, the concentration of the acetate being at least about 0.22 normal, the available hydrogen ion concentration being, however, at least 2.8 normal.

9. The process of claim 8 wherein the temperature of treatment is maintained at between about room temperature and 210 F.

. References Cited in the file of this patent UNITE]? STATES PATENTS 

1. A COMPOSITION OF MATTER ADAPTED FOR CHEMICALLY MILLING METALS WHICH CONSISTS ESSENTIALLY OF AN AQUEOUS ACIDIC MEDIUM CONTAINING A NITRATE IN THE CONCENTRATION RANGE OF ABOUT 0.1-4.7 MOLAR, A CHLORIDE IN THE CONCENTRATION RANGE OF ABOUT 0.1-2.2 MOLAR, A FLUORIDE IN THE CONCENTRATION RANGE OF ABOUT 0.25-5.3 MOLAR, AND AN ACETATE IN A CONCENTRATION OF AT LEAST 0.22 NORMAL, THE AQUEOUS MEDIUM AS A WHOLE HAVING AN AVAILABLE HYDROGEN ION CONCENTRATION IN THE CONCENTRATION RANGE OF ABOUT 2.8-10.7 MOLAR.
 2. A PROCESS FOR CHEMICALLY MILLING A METAL SELECTED FROM THE GROUP CONSISTING OF ALUMINUM, TITANTIUM, STEEL AND ALLOYS OF THESE METALS, SAID PROCESS COMPRISING SUBJECTING THESE METALS TO THE ACTION OF A COMPOSITION OF MATTER WHICH CONSISTS ESSENTIALLY OF AN AQUEOUS ACIDIC MEDIUM CONTAINING A NITRATE IN THE CONCENTRATION RANGE OF ABOUT 0.1-4.7 MOLAR, A CHLORIDE IN THE CONCENTRATION OF ABOUT 0.1-2.2 MOLAR, A FLUORIDE IN THE CONCENTRATION RANGE OF ABOUT 0.25-5.3 MOLAR, AND AN ACETATE IN A CONCENTRATION OF AT LEAST 0.22 NORMAL, THE AQUEOUS MEDIUM AS A WHOLE HAVING AN AVAILABLE HYDROGEN ION CONCENTRATION IN THE CONCENTRATION RANGE OF ABOUT 2.8-10.7 MOLAR. 